A large‐scale field experiment of artificially caused landslides with replications revealed the response of the ground‐dwelling beetle community to landslides

Abstract Precipitation‐induced landslides, which are predicted to increase under the changing climate, may have large impacts on insect community properties. However, understanding of how insect community properties shift following landslides remains limited because replicated research involving landslides, which are large‐scale disturbances with stochastic natural causes, is difficult. To tackle this issue, we conducted a large‐scale field experiment by artificially causing landslides at multiple sites. We established 12 landslide sites, each 35 m × 35 m, and 6 undisturbed sites in both planted and natural forests and collected ground‐dwelling beetles 1 year later. We found that forest type (i.e., pre‐disturbance vegetation) did not affect the structure of a ground‐dwelling beetle community disturbed by a landslide (landslide community), but the structure of an undisturbed community was affected by forest type. Moreover, the structures of landslide and undisturbed communities were completely different, possibly because landslides create harsh environments that act as an ecological filter. Thus, a niche‐selection process may have a critical role in community assembly at landslide sites. There were no significant differences in species diversity between undisturbed and landslide communities, suggesting that landslides to not reduce species richness overall. However, among‐site variability in species composition was much greater at landslide sites than at undisturbed sites. This result suggests that stochastic colonization predominated at the landslide sites more than undisturbed sites. Synthesis and applications. Overall, our results suggest that both deterministic and stochastic processes are critical in community assembly, at least in the early post‐landslide stage. Our large‐scale manipulative field experiment with replications has thus resulted in new insights into biological community properties after a landslide.


| INTRODUC TI ON
Large-scale disturbances caused by global climate change and anthropogenic environmental modification have profound impacts on biodiversity and the provision of vital ecosystem services around the world (Aavik et al., 2021;Barnosky et al., 2012;Seidl et al., 2017). In recent years, the frequency and severity of large-scale disturbances have increased globally (Patacca et al., 2023;Seidl et al., 2017;Westerling, 2016). While natural disturbances have historically impacted biomes and can contribute to the maintenance of biodiversity (Schowalter, 2012), recent changes in disturbance regimes may result in unprecedented biodiversity loss (Bowd et al., 2023;Johnstone et al., 2016). However, our understanding of the general principles governing the responses of ecological communities to the recent large-scale disturbances remains incomplete. This knowledge could lead to the development of appropriate conservation and ecosystem management practices as an urgent issue for the maintenance of biodiversity, ecosystem functions, and a sustainable society.
One type of large-scale disturbance that may increase in frequency because of global climate change is a landslide, which can be triggered by extreme precipitation (Gariano & Guzzetti, 2016;Sidle & Bogaard, 2016). Landslides greatly reduce the quality of biological habitats through changes in both topography and environmental factors, including the loss of huge amounts of surface soil and vegetation (Highland & Bobrowsky, 2008). On the other hand, at local scale, landslides may create habitat heterogeneity, thereby contributing to the maintenance of biodiversity (Geertsema & Pojar, 2007;Remelli et al., 2019). Understanding of how ecological community properties such as species composition and species diversity shift following landslides is still limited for two main reasons.
First, most previous studies of the impacts of landslides have focused primarily on plant communities (Guariguata, 1990;Walker et al., 1996). For example, the process of vegetation recovery after landslides has been monitored (Chen et al., 2020;Yang et al., 2018), and how environmental factors such as soil conditions affect the post-landslide vegetation recovery rate has been examined (Lin et al., 2005;Shiels et al., 2008). A very few studies have examined non-plant communities such as arthropods (Hao-Chiang et al., 2017), which may rapidly colonize and alter biophysical conditions of landslide-degraded habitats in advance of plant colonization. For example, ground-dwelling beetles can influence soil quality though relocating waste by vertebrates and transporting fungi (Nichols et al., 2008;Vašutová et al., 2019). In this regard, it should be noted that arthropods, the most diverse eukaryotic group on Earth, have a critical role in ecosystem functioning (Prather et al., 2013). Second, because a landslide is a large-scale disturbance caused by a stochastic natural event such as an earthquake or heavy rainfall (Sidle & Bogaard, 2016), studies of the impacts of landslides suffer a methodological limitation. Each study has typically been conducted in a single disturbed locality, each with different geographic and environmental conditions (e.g., climate and vegetation), without replications or experimental treatments (Schowalter, 2012). As a result, general patterns in how community properties shift in response to landslides and in how environmental factors influence post-landslide community assemblages are poorly understood. To address these problems, large-scale field experiments with replications of artificially caused landslides are needed that examine a wide range of ecological communities, in addition to vegetation communities.
Although stochastic and deterministic processes are not mutually exclusive, their relative importance varies over time and space under different environmental conditions (Chase, 2010;Chave, 2004;Leibold & McPeek, 2006). Chase (2007) showed that, in harsh environments, niche selection predominantly filters out species rather than the stochastic process of ecological drift. Thus, the environmental harshness that follows landslides can be expected to result in niche-assembled communities. Although pre-disturbance vegetation may affect post-disturbance community assemblages because of remaining propagules (Bergeron et al., 2017;Johnstone et al., 2016), this effect may be weaker following a landslide than after other types of disturbances such as clear-cutting and forest fires because both natural and artificial landslides often completely remove the surface soil along with any propagules and insect eggs or larvae.
In this study, we examined the community assembly response to replicated experimental landslides. We conducted a large-scale field experiment with replications of artificially caused landslides and investigated communities of ground-dwelling beetles, which may rapidly colonize a disturbed area regardless of whether all vegetation is removed. Ground-dwelling beetles are useful bioindicators because of their quick response to environment change (Rainio & Niemelä, 2003). Further, some of ground-dwelling beetles are known as seed predators and can mediate seed dispersal (Ali et al., 2022;de Vega et al., 2011;Griffiths et al., 2016;Müller et al., 2022), and even some fungi are dispersed by ground-dwelling beetles (Heitmann et al., 2021;Vašutová et al., 2019). Moreover, plant growth enhancement by some species of ground-dwelling beetles through improving soil quality was reported (Nichols et al., 2008).
Thus, because the initial short-term responses of ground-dwelling beetles to disturbances may affect the subsequent succession of plants and other taxa, general insights into their initial responses gained by our experimental approach can provide a basis for ecological restoration. In addition, our experimental approach allows us to test the effects of the assembly process (i.e., niche selection or ecological drift) and the effects of pre-disturbance vegetation (i.e., natural or planted forest) following a disturbance realistically mimicking a landslide disturbance in natural ecosystems. Specifically, we address the following questions: (1) How does forest type affect the beetle community structure under undisturbed conditions and post-landslide conditions? (2) Do landslide environments act as a filter, resulting in a different community structure between landslide and undisturbed conditions? (3) Does the occurrence of a landslide reduce species diversity and among-site variability of communities of ground-dwelling beetles? 2 | MATERIAL S AND ME THODS

| Landslide experiment
This study was conducted in the Teshio, Nakagawa, and Uryu Experimental Forests of Hokkaido University in northern Hokkaido, Japan. The dominant tree species in these cool-temperate zone experimental forests are birch (Betula ermanii, and Betula platyphylla var. japonica), Japanese oak (Quercus mongolica var. grosseserrata), painted maple (Acer mono), and Sakhalin fir (Abies sachalinensis). In natural forests, the understory is dominated by dwarf bamboo (Sasa senanensis). In each experimental forest (hereafter referred to as "locality"), we established four landslide treatment sites, consisting of three natural forest sites and one site in a planted forest of A. sachalinensis ( Table 1). We also established two undisturbed sites as controls in each experimental forest; one was in a natural forest and the other was in a planted forest of A. sachalinensis (Table 1). Thus, a total of 12 landslide treatment sites and 6 undisturbed sites were established in the three localities.
Tree composition was assessed prior to experimental landslide treatment. In natural forest sites, the average basal area proportions were as follows: 19% birch, 18% oak, 18% fir, 14% linden, 12% maple, and 19% others. The forest floors were covered with dwarf bamboo, in which the coverage was more than 90%. Structures of all natural forests for the landslide and undisturbed sites were similar across localities. In all planted forest sites, more than 90% of trees were the fir A. sachalinensis. Their forest floors were also dominated by dwarf bamboo, but the cover was around 10%. At each landslide treatment site, all trees were cut and removed from a 35 m × 35 m area which was equally divided into four plots between December 2019 and March 2020, and in summer 2020, understory vegetation, roots, and surface soil were almost completely removed with bulldozers ( Figure 1).
Slope gradients after the landslide treatment ranged from 11° to 26° (20.4 ± 4.5°, mean ± SD, Table 1). In nature, landslides simultaneously create both erosional and depositional zones (Cruden & Varnes 1996;Geertsema & Pojar, 2007). This experimental treatment primarily mimicked the erosional zone of a landslide, which could be one of the most different characteristics from other disturbances. Additionally, together with the surface soil, above and belowground part of the understory vegetation, as well as the roots of overstory trees, were deposited in the area beneath each site, resembling a natural landslide depositional zone, although logs and trunks were removed from the experimental sites. This study primarily focused on ground-dwelling beetle communities within the erosional zone that was artificially created. At each undisturbed site, a total of approximately 100 m 2 was equally divided into four plots with at least 3 m distance separating each plot in natural and planted forests. All undisturbed sites were approximately 100 m away from the nearest treatment site and had not been disturbed artificially in at least the past 50 years.

| Community census of grounddwelling beetles
We investigated ground-dwelling beetles 1 year after the landslide treatment. For each plot (i.e., four plots in one landslide or undisturbed site), we created a two-by-three grid that equally divided a plot. We selected five intersections out of six intersections of the grid (i.e., approximately 4.5-5.5 m spacing between traps) and assigned each of five pitfall traps (69 mm in diameter 97 mm deep) to TA B L E 1 Description of study sites. and abundance data were recorded ( Table 2).
In addition to species composition and species diversity, we also examined body size of beetles in landslide and undisturbed sites as one of community properties. Digital images of all individuals of the collected beetles were taken with a digital camera (Tough TG-6; Olympus). Then, the lengths of the head, pronotum, and elytron were measured, using ImageJ 1.53k. The sum of these lengths was used as body size.

| Statistical analyses
We used Hill numbers (q = 0, 1, 2) to construct individual-based rarefaction and extrapolation curves with 95% confidence intervals to compare species diversity of undisturbed communities between planted and natural forests. permutations was performed based on the dissimilarity to examine the effects of disturbance treatment on community structure.
Among-site variability in composition can be partitioned into spatial species turnover and nestedness of assemblages, which result from species replacement and species loss from site to site, respectively (Baselga, 2010). We partitioned among-site variability in landslide community composition into species turnover and nestedness. In those comparisons of species diversity, composition, and variability between landslide and undisturbed communities, we combined the dataset of both planted and natural forest sites.
Body size data were fitted using a liner mixed model with treatment as a fixed effect and site as a random effect. The dataset of both planted and natural forest sites was merged. The significant test was performed, using a F-test with Kenward-Roger approximation.

| Species composition and abundance of the collected ground-dwelling beetles
In total, we collected 587 (including six unidentified) individuals belonging to 31 species of ground-dwelling beetles across all sites (

| Effects of forest type on undisturbed and landslide communities
The individual-based rarefaction and extrapolation curves indicated no significant differences in species diversity of undisturbed Abbreviations: NAT_L, landslide treatment in natural forest (n = 9); NAT_U, undisturbed natural forest (n = 3); PLA_L, landslide treatment in planted forest (n = 3); PLA_U, undisturbed planted forest (n = 3).
communities between planted and natural forests for q = 0; thus, they did not differ in species richness. However, for q = 1 (Shannon diversity) or 2 (Simpson diversity), species diversity was greater in planted forests (Figure 2).
The magnitude of among-site variability in species composition was similar between undisturbed planted and natural forests ( Figure 3a; Permutation test, F = 0.08, p = .76).
Effects of locality, forest type, and month on community composition were significant at undisturbed sites ( Table 3). In contrast, no significant effect of forest type (i.e., pre-disturbance vegetation) on landslide communities was detected (Table 3, Figure 3b). Thus, predisturbance vegetation did not influence the community structure of ground-dwelling beetles after a landslide disturbance.

| Comparison of structure and species diversity between undisturbed and landslide communities
The individual-based rarefaction and extrapolation curves indicated no significant differences in species diversity between undisturbed and landslide communities ( Figure 4).
The PCoA results illustrated that not only community composition but also among-site variability clearly differed between landslide and undisturbed sites. Among-site variability was much greater at landslide sites than at undisturbed sites ( Figure 5; Permutation test, F = 14.67, p = .001). The average distance from the centroid F I G U R E 2 Individual-based rarefaction curves (solid lines) and extrapolation curves (dashed lines) of species diversity in undisturbed communities in natural (NAT) and planted (PLA) forests based on three orders of Hill numbers: (a) q = 0 (species richness), (b) q = 1 (Shannon diversity), and (c) q = 2 (Simpson diversity). The body size of ground-dwelling beetles collected in landslide sites was significantly smaller than in undisturbed sites ( Figure 6;

F I G U R E 3
F-test, p = .015). When beetle species identity as a random effect was added to the model, effect of landslide treatment on body size was not significant (p = .79). This indicates the difference in body size was due to community composition rather than intraspecific variation.

| DISCUSS ION
This study clearly illustrated community assembly of grounddwelling beetles in response to landslide disturbance. Although pre-disturbance vegetation did not affect the community structure after landslides, species composition greatly differed between landslide and undisturbed communities. These results indicate that a niche-selection process shaped the ground-dwelling beetle communities that colonized sites after a landslide. Furthermore, among-site variability in species composition (i.e., β-diversity) was much greater at landslide sites, whereas species diversity was comparable between landslide and undisturbed sites. Overall, this study suggests that a stochastic process of ecological drift, as well as niche selection, is critical for community assembly after a landslide.

| The structure of undisturbed ground-dwelling beetle communities
To clarify the structure of the communities disturbed by landslides, the fundamental structure of the undisturbed communities must be understood as a comparison. Therefore, we first describe the characteristics of the undisturbed forest communities.
The structure of undisturbed forest communities, but not that of landslide-disturbed communities, differed by forest type (planted or natural; Table 3). The species richness of undisturbed grounddwelling beetle communities was comparable between planted and natural forests (Figure 2a), but the evenness was greater in planted forests (Figure 2a,b). This greater evenness in planted forests is likely because planted forests are a more stable and homogeneous TA B L E 3 Effects of locality, forest type, and month on the ground-dwelling beetle community.

F I G U R E 4
Individual-based rarefaction curves (solid lines) and extrapolation curves (dashed lines) of species diversity in landslide and undisturbed communities based on three orders of Hill numbers: (a) q = 0 (species richness), (b) q = 1 (Shannon diversity), and (c) q = 2 (Simpson diversity). environment than natural forests. Greater environmental variability in terms of temperature, intensity of ambient insolation, and wind speed may decrease evenness in arthropod communities (Larrivée & Buddle, 2009). Previous studies have reported that carabid beetle communities have lower species richness in conifer plantations than in natural deciduous broadleaf forests regardless of the age of plantations (5-50 years) (Magura et al., 2003;Yu et al., 2006 (Kaizuka et al., 2020), at undisturbed planted forest sites ( Table 2). In addition, conifers (A. sachalinensis) often occur in the natural forest.
Thus, beetle species may frequently be dispersed across natural and planted forest patches in a continuous, mixed-forest landscape, and this dispersal would maintain comparable levels of species diversity in the beetle communities between planted and natural forest sites.
These explanations were not mutually exclusive. The similar amongsite variability in beetle species composition between the planted and natural forest sites may also support the existence of a metacommunity where species dispersal occurs across both the uniform environment of planted forest patches and heterogeneous natural forest environments. In addition, the differences in community composition between the planted and natural forest sites were not large (as shown by the considerable overlap in Figure 3a).

| Effect of pre-disturbance vegetation on the landslide communities
To our knowledge, this is the first study to demonstrate experimentally that pre-disturbance vegetation had no effect on the structure of communities disturbed by landslides (Table 3, Figure 3b). Iida et al. (2021) showed an effect of pre-disturbance vegetation on the arthropod community structure after a volcanic eruption; they suggested that the presence or absence of post-disturbance litter deposition, which varies with the pre-disturbance vegetation, affects the community structure. In contrast, we observed very little litter deposition at our landslide sites because only 1 year had passed since the complete removal of the surface soil. This lack of litter deposition may explain why the pre-disturbance vegetation had no effect on the structure of the ground-dwelling beetle communities in our data. However, other taxa, in particular plant taxa, may be affected by the pre-disturbance vegetation during the community assembly process because of seed and propagule dispersal from the surrounding vegetation (i.e., pre-disturbance vegetation).
Therefore, it is likely that during the vegetation recovery stage, the ground-dwelling beetle community would also be indirectly affected by the pre-disturbance vegetation via the recovered vegetation.
No differences in landslide communities between pre-vegetation types would result from immigration processes in assembly. Most of species were only collected from either landslide or undisturbed sites, whereas a few common species were collected from both sites ( Table 2). This suggests two processes in immigration of grounddwelling beetles to landslide sites: (1) common species immigrating from neighboring forests, (2) species immigrating from habitats that are similar to our landslide sites. For the former, a few dominant species, which inhabit both planted and natural forest areas, colonized the landslide sites ( Table 2). For the latter, many species with a high dispersal capacity, which are likely to prefer the landslide-like habitats such as non-forested gravel environments (e.g., the tiger beetle C. sachalinensis and the click beetle Y. aeneoniger), colonized the landslides sites. However, the majority of species were from the latter group. This is likely to be because landslides created a harsh environment characterized by higher temperatures, lower humidity, and stronger sunlight compared with the neighboring forests. Therefore, niche selection probably plays a critical role in community assembly at landslide sites, as discussed below.

| Niche selection and stochasticity in response to landslides
Previous studies have reported that species richness of grounddwelling beetles increases in communities immediately after disturbances such as windthrows (Bouget, 2005), forest fires (Paquin, 2008), and clear-cutting (Heliölä et al., 2001;Koivula et al., 2002;Yu et al., 2006). Our experimental study yielded similar results in that the overall species diversity of ground-dwelling beetles was not decreased in the early recovery stage (i.e., 1 year after landslides) when landslide communities were compared with undisturbed communities.
However, the fact that the species composition of the communities differed significantly between post-landslide and undisturbed sites suggests that ecological filtering from the regional species pool likely differed between the landslide and undisturbed forest sites.
The absence of a forest-type effect on landslide communities, despite the existence of such an effect on the undisturbed community structure, supports this interpretation. Therefore, niche selection has a critical role in shaping landslide communities of grounddwelling beetles. In the mixed forests of northern Hokkaido, there are many sites with habitats similar to our landslide sites, including natural small-landslide sites, gravel river banks, and sites that have been artificially disturbed by logging and soil scarification (Aoyama et al., 2011). Because biotic and abiotic environments in these habitats may be similar to those of landslide sites (e.g., no vegetation, bare surface soil, or gravel), beetle populations in these habitats are likely to be sources of newly colonizing beetles, such as C. sachalinensis, Bembidion paediscum. Cicindela sachalinensis, and B. paediscum, which are known to prefer forest road, bare surface soil, and gravel river bank habitats (Morimoto et al., 2007;Ueno et al., 1985). Patchy environments similar to our landslide habitats exist widely within a natural forest, and dispersal from the metacommunity around these landscapes is not restricted due to natural forest matrix. Therefore, beetle species from the metacommunity can immediately colonize landslide sites and maintain species diversity, with niche selection having a strong effect due to the harsh environmental conditions.
On the other hand, the among-site variability was 4.1 times higher for landslide sites than for undisturbed sites because of high species turnover. In fact, many of the species collected at the landslide treatment sites were collected from only a few sites, and no species were found across all landslide sites. The high site-to-site variability and high turnover suggest that stochastic colonization by species filtered by the harsh post-landslide environment predominantly affected community structure at the landslide sites. Previous studies have reported an increase in the number of species with small body size and high dispersal capacity in arthropod communities after a disturbance (Bailey et al., 2018;Butterfield et al., 1995;Rainio & Niemelä, 2003). In our experiment, the body size of the beetle community was also smaller at landslide sites than at undisturbed sites ( Figure 6), suggesting colonization by beetle species with a high dispersal capacity. Thus, beetle species with a high dispersal capacity inhabiting similar types of habitats (e.g., gravel river banks, logging sites, and forest roads) are likely to immigrate to the scattered landslide sites in a stochastic manner. Alternatively, the different environmental filter due to the difference in the environment at each landslide site may explain the high site-to-site variability and high species turnover. However, the site-to-site variability in undisturbed communities was much lower than landslide sites while environmental heterogeneity among undisturbed sites may be greater than landslide sites. Thus, differences in environmental conditions among landslide sites would unlikely explain the large community variability. Overall, these results suggest that the landslide treatment not only caused deterministic processes but also strengthen stochastic processes in community assembly in comparison with undisturbed sites, at least in the early post-landslide stage.
Previous studies have shown that patch connectivity and habitat heterogeneity are important for the maintenance of ground-dwelling beetle populations (Duflot et al., 2014;Neumann et al., 2016;Niemelä, 2001). Our results also likely reflect both the presence of environments such as river banks and logging sites around our study sites (i.e., habitat heterogeneity) and connectivity between such sites and our experimental sites. In other words, the surrounding landscape structure, including the degree of patch connectivity, the distance between patches, and the diversity of patch environments (i.e., the number of different insect habitat types) may strongly influence the recovery rate of species diversity after a disturbance.
This study yielded new insights into community assembly by comparing communities between landslide and undisturbed treatments at replicate sites in a large-scale field experiment. However, the results show only the immediate responses of the beetle communities. In the future, further temporal changes in the communities at these sites need to be investigated. In addition, the dispersal abilities of the species that colonize the landslide sites should be investigated. Further, the contributions of patch connectivity, patch diversity, and patch distribution to the recovery of species diversity after a landslide should be quantified. For achieving these aims, large-scale manipulative field experiments with replicate sites have much to offer.

CO N FLI C T O F I NTER E S T S TATEM ENT
The authors declare that they have no conflict of interest.

O PEN R E S E A RCH BA D G E S
This article has earned an Open Data badge for making publicly available the digitally-shareable data necessary to reproduce the reported results. The data is available at https://doi.org/10.6084/ m9.figsh are.22210723.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are openly available in figshare at https://doi.org/10.6084/m9.figsh are.22210723.